1. Field of the Invention
This invention relates to a deionization apparatus, more particularly to an electrical deionization apparatus capable of efficient and consistent deionization of water over a wide range of ion concentrations.
2. Discussion of the Related Art
In the technology of separating aqueous solutions into the solvent and the solute, separating the solute which accounts for only a small portion of the solution is theoretically more energy-saving than separating the major-component solvent, as is obvious to the skilled artisan.
This difference is also-reflected in methods of removing ions that are contained in small amounts in liquids and they can be classified into two groups, the first group being intended to remove the solvent water and comprising distillation and reverse osmosis, and the second group being for removing the solute ions and comprising ion exchange and electrodialysis.
Distillation is a method for causing changes in the phase of water by heating and cooling cycles, and reverse osmosis is a method for pressurizing water with a high-pressure pump so that it passes through a permeable membrane. Both methods are energy-intensive approaches.
Ion exchange is a method using an ion-exchange resin that causes selective exchange and adsorption of ions in liquids. In this method, acids or alkalis are used as a regeneralizer of the ion-exchange resin and must be handled with great care. The corrosion of the equipment and leakage due to the regeneralizer are other necessary considerations. Another requirement is the treatment of the liquid waste resulting from the regeneration step.
Electrodialysis uses an electrodialyzer in which cation-exchange membranes are alternated with anion-exchange membranes to make an alternate arrangement of concentration compartments and deionization compartments between electrodes. With a gradient in electrical potential used as the driving force, the ions in a liquid are separated by selective movement from the deionization compartment through the ion-exchange membranes into the concentration compartments. Although electrodialysis permits continuous operation without using any chemicals, its applicability has been limited for the following reasons: a current must be applied in an amount sufficient to transport the ions of interest; if the percent salt removal is to be increased, hardness components are prone to precipitate at the interface with the ion-exchange membrane, making it impossible to produce deionized water of high specific resistance (i.e., high purity); and if the feed solution has low ion concentration, a higher voltage is required to transport the ions.
Under these circumstances, it has generally been held that solutions of high salt concentrations can advantageously be deionized by reverse osmosis, solutions of lower salt concentrations by electrodialysis, and solutions of even lower salt concentrations by ion exchange.
An electrical regenerable deionization apparatus and method that fills an ion exchanger between ion-exchange membranes in the deionization compartment of an electro-dialyzer and which enables more efficient deionization to yield products of higher purity than achievable by the conventional electrodialysis method was first proposed by Paul Kollsman (Japanese Patent Publication Nos. 1859/1958 and 4681/1959). However, for more than 25 years after his proposal, no reliable apparatus of this model has been offered for operation on a commercial scale.
Nevertheless, primarily being motivated by the improvements in the performance of ion-exchange membranes, the advances in pre-treatment methods, the demand of the industrial sector for a deionization apparatus that does not need complicated regeneralizing facilities, and a global concern for the saving of resources and energy, efforts have been constantly made to develop a practically feasible electrical deionization technology that fills the ion exchanger between ion-exchange membranes and many salient improvements have been proposed in recent years.
Some of these improvements are commercially applicable and include the following: a method in which the deionization compartment is limited in terms of structural parameters such as width and thickness and filled with a mixture of cation- and anion-exchange resins (Japanese Patent Publication No. 72567/1992); a method in which the interior of the deionization compartment is segmented and filled with anion- and cation-exchange membranes alternately (Japanese Patent Public Disclosure No. 71624/1992); a method in which the deionization compartment is filled with a mixture of cation-exchange fiber, anion-exchange fiber and inactive synthetic fiber (Japanese Patent Public Disclosure No. 236889/1995); a method in which the deionization compartment is filled with a mixture of cation/anion-exchange resins and cation/anion-exchange fibers (Japanese Patent Public Disclosure No. 277344/1993); and a method in which a multi-core composite fiber of sea-island pattern having cation-exchange groups introduced therein is mixed with a multi-core composite fiber of the same pattern having anion-exchange groups incorporated therein and the mixture is shaped for filling (Japanese Patent Public Disclosure No. 192163/1996). These methods share the common feature of combining the mixed-bed ion-exchange resin technology (MB method) with electrodialysis.
However, all of these proposals have had difficulty in producing deionized water of high purity consistently over a prolonged time because they have one or more of the following problems: in order to prevent short-circuiting by the feed stream, the deionization compartment must be closely filled with the ion exchangers by a cumbersome procedure; due to the close filling, the pressure of the stream flowing into the deionization compartment must be held high; the variations in the flow of the water being fed into the deionization compartment may potentially disrupt the homogeneity of the mixed ion exchangers; ion-exchange resins of a rigidly cross-linked structure may become disintegrated during service; since the oppositely charged ion exchangers arranged in the direction of ion migration retard the smooth transport of ions, the ion exchangers become gradually xe2x80x9cloadedxe2x80x9d as the operation proceeds, potentially resulting in incomplete deionization; the ion exchangers must be mixed uniformly by a cumbersome procedure; it is difficult to secure the strength of the shaped ion exchanger; it is cumbersome to control the porosity of the shaped ion exchanger; and it is difficult to clean the ion exchangers sufficiently to prevent the dissolution of organic carbon (TOC).
The structure of the conventional electrodialyzer is characterized in that the spacer secures the necessary passageway for the feed stream, that it can be operated at low in-flow pressure and that no ion exchangers that interfere with the movement of ions are provided in the direction of ion migration. Continued attempts have also been made to design an advanced type electrodialyzers by filling the deionization compartment with an ion-conducting spacer so that power consumption is reduced while retaining the advantageous features of that structure. Although several tens of percent of cut on power consumption has been demonstrated, those attempts could not reach the stage of commercialization because of the following reasons: due to the difficulty in controlling the chemical reaction involved in introducing ion-exchange groups into the spacer material, mass production of the spacer is difficult; it is also difficult to secure the strength of the spacer; difficulty is also encountered in suppressing the dissolution of TOC from the spacer. Particularly in the case where the deionization compartment is filled with the ion-conducting spacer alone in place of ion-exchange resins, the spacer which, as an ion exchanger, has a smaller surface area than the ion-exchange resins makes only insufficient contact with the ions in the deionization compartment and the water to be treated flows as if it xe2x80x9cshort-circuitsxe2x80x9d the interior of the deionization compartment, thus failing to be deionized with high efficiency. The group of advanced type electrodialyzers have had a strong need for a spacer that has comparable capabilities and strength to the heretofore commonly employed polypropylene or polyethylene diagonal net spacers, that has ion conductivity and that is suitable for use in apparatus of industrial scale. As far as the present inventors know, there has been no example of an electrical deionization apparatus in which a non-woven fabric having ion-exchange capabilities is combined with an ion conducting spacer.
FIG. 4 shows an example of the prior art electrical deionization apparatus filled with ion-exchange resins.
The electrical deionization apparatus shown in FIG. 4 comprises, in order from the cathode side, a cathode 1, an anion-exchange membrane 2, a concentration compartment 3, a cation-exchange membrane 4, a deionization compartment 5, an anion-exchange membrane 2xe2x80x2, a concentration compartment 3xe2x80x2, a cation-exchange membrane 4xe2x80x2 and an anode 10, these being arranged in the order written. If necessary, a plurality of deionization compartments may alternate with a plurality of concentration compartments in a parallel array between the two electrodes. The deionization compartment 5 is filled with a mixture of cation- and anion-exchange resins. To operate the apparatus, voltage is applied between the anode 10 and the cathode 1 while, at the same time, water to be treated 11 is fed into the deionization compartment 5 and concentration water (feed water into concentration compartment) streams 14 and 14xe2x80x2 are fed into the concentration compartments 3 and 3xe2x80x2, respectively. When the water to be treated and the concentration water are thusly introduced, the cations and anions in the water are respectively attracted toward the cathode and anode; since cation-exchange membranes are selectively permeable to cations whereas anion-exchange membranes are only permeable to anions, the cations in the feed water (e.g., Ca2+ and Na+) undergo ion-exchange on the cation-exchange resin filled in the deionization compartment 5 and pass through the cation-exchange membrane 4 to enter the concentration compartment 3 whereas the anions (e.g., Clxe2x88x92, SO42xe2x88x92, HSiO3xe2x88x92 and CO32) undergo ion-exchange on the anion-exchange resin in the deionization compartment 5 and pass through the anion-exchange membrane 2xe2x80x2 to enter the concentration compartment 3xe2x80x2. On the other hand, the movement of anions from the concentration compartment 3 to the deionization compartment 5 and that of cations from the concentration compartment 3xe2x80x2 to the deionization compartment 5 are blocked since cation-exchange membranes are not permeable to anions and anion-exchange membranes are not permeable to cations. As a result, product water 12 having its ion concentration lowered is obtained in the deionization compartment 5 and concentrate water streams 13 and 13xe2x80x2 having increased ion concentrations are obtained in the concentration compartments 3 and 3xe2x80x2. In the deionization compartment, the ion concentration of water decreases as it flows down toward the bottom. In the neighborhood of the interface between different types of ion-exchange resins, water dissociates (H2Oxe2x86x92H++OHxe2x88x92) to have the ion-exchange resin regenerated continuously, thus allowing for continuous trapping of ions in the deionization compartment.
In the deionization compartment 5, ions also move through the water being treated but most of them move through the ion-exchange resin of the same type (cations transfer through the cation-exchange resin and anions through the anion-exchange resin) and smooth ion transfer does not occur unless ion-exchange resins of the same type communicate in series with each other. As a result, the performance for ion removal is by no means consistent, nor is it sufficiently high that water of a purity comparable to that of water treated by RO (reverse osmosis) to remove hardness components can be continuously deionized to an even higher purity. It has also been difficult to treat filtered water having a higher ion concentration or deionize high-purity water of low ion concentration to produce ultra-pure water.
With a view to solving these problems, an improvement of the electrical deionization apparatus having an ion exchanger filled in the deionization compartment was proposed (see Japanese Patent Public Disclosure No. 99221/1997). The proposal concerned the use of a non-woven fabric (polymerized fiber) as the ion exchanger and a cation-exchange non-woven fabric and an anion-exchange non-woven fabric were placed in a face-to-face relationship, and spaced apart by a synthetic resin net that was used in the conventional electrodialyzer. The ion exchanger non-woven fabrics had ion-exchange groups concentrated at high density on the surface of fiber and, hence, were advantageous for trapping ions. When ion exchanger non-woven fabrics having a larger surface area than ion-exchange membranes were used, the efficiency of trapping ions as the feed water was passing through the deionization compartment could be considerably enhanced. What is more, in the proposed apparatus, the ion exchanger non-woven fabrics were in close contact with the ion-exchange membranes and, hence, the ions trapped on the ion exchanger non-woven fabrics could smoothly transfer to the ion-exchange membranes, through which they permeate to enter the concentration compartments.
With this apparatus, the feed water need not always be passed through a closely filled layer of ion exchanger, making it possible to lower the pressure loss, simplify the ion exchanger packing operation and reduce the complexity of the apparatus. As a result, an electric deionization apparatus could be realized that solved the various problems with the prior art and which was capable of maintaining the high purity of the product water over an extended period.
In fact, however, if one tries to perform further enhanced deionization of water using the proposed apparatus having the ion :exchanger non-woven fabrics and the synthetic resin net filled in the deionization compartment, the cathode-to-anode voltage is increased and more energy is consumed.
In particular, if one wants water quality comparable to ultra-pure water, the product water is substantially free of residual ions and, in areas close to the synthetic resin net, it becomes almost like an insulator and a very high voltage is needed to continue the operation; this is certainly an economic disadvantage from a facilities viewpoint.
The apparatus had another problem. As the ion concentration in the deionization compartment decreased, that of the concentrate water increased and the amount of ion diffusion due to the difference in ion concentration between the concentrate water and the deionized water (namely, the amount of ion transfer from the concentrate water to the deionized water) also increased to balance with the amount of ion transfer due to the potential difference (namely, the amount of ion transfer from the deionized water to the concentrate water). In order to obtain product water of high purity in that situation, the ion concentration in the concentration water had to be lowered.
It is known that the ion concentration in the concentration water that is appropriate for achieving efficient ion transfer is no more than 200 times, desirably no more than 100 times, the ion concentration in the product water. Therefore, if one wants to obtain ultra-pure water having a specific resistance on the order of 18 Mxcexa9xc2x7cm, water deionized to a specific resistance of at least 5 Mxcexa9xc2x7cm is desirably provided at the entrance for the concentration water.
On the other hand, if the ion concentration of the concentration water is lowered, the electrical resistance of the concentration compartment increases to consume more power. Therefore, in order to reduce power consumption, it was proposed to increase, rather than decrease, the ion concentration of the concentration water (see Japanese Patent Public Disclosure Nos. 24374/1997 and 290271/1997).
The present invention has been accomplished under these circumstances and has as an object providing an electrical deionization apparatus that can be operated at such low cathode-to-anode voltages that it finds extensive use in various applications ranging from the deionization of filtered water of high ion concentration to the production of ultra-pure water and which allows for an increase in capacity.
The present inventors conducted intensive studies in order to attain the stated object in connection with an electrical deionization apparatus that fills the deionization compartment with an ion exchanger consisting of ion-exchange woven fabrics or non-woven fabrics, with a cation-exchange woven or non-woven fabric being placed in a face-to-face relationship with an anion-exchange woven or non-woven fabric as they are spaced apart by a synthetic resin net. As a result of their studies, the present inventors found that the performance of the apparatus could be dramatically improved by functionalizing an ion-exchanging capability to the synthetic resin net so that it works as an ion-conducting spacer. The present invention has been accomplished on the basis of this finding.
Thus, the present invention relates to an electrical deionization apparatus in which at least part of cation-exchange membranes and anion-exchange membranes alternate between electrodes to form an alternating array of deionization and concentration compartments and the deionization compartment contains a woven or non-woven fabric made of cation-exchange fiber that is placed on the cation-exchange membrane side in a face-to-face relationship with a woven or non-woven fabric made of anion-exchange fiber that is placed on the anion-exchange membrane side, with the passageway of feed water between the two woven or non-woven fabrics containing an ion-conducting spacer provided with; an ion-exchanging capability.
The ion-exchange fiber to be used in the electrical deionization apparatus of the invention is preferably obtained by grafting ion-exchange groups to a substrate made of polymeric fiber. The substrate may consist of monofilaments of a single type of fiber; alternatively, it may consist of composite fiber having the core and the sheath made of different polymers. An example of the composite fiber that can be used is one having a core-sheath structure in which the sheath component is made of a polyolefinic polymer such as polyethylene and the sheath component is made of other polyolefinic polymer such as polypropylene. Ion-exchange groups may be introduced into such composite fiber material by radiation-induced graft polymerization and the product is preferred for use as an ion-exchange fiber material in the invention since it has high ion-exchanging performance and is available in uniform thickness.
To prepare the ion-conducting spacer for use in the invention, a diagonal net made of a polyolefinic high-molecular weight resin, for example, polyethylene conventionally used in electrodialyzers may be used as a substrate and subjected to radiation-induced graft polymerization to functionalize an ion-exchanging capability. The thus obtained product is preferred in the invention since it has high ion conductivity and allows for efficient distributing of the feed water.
In the deionization apparatus of the invention, the deionization compartment has preferably a thickness of 2.5-5 mm and the concentration compartment including an electrode compartment has preferably a thickness of 0.5-2.0 mm. Preferably, the deionization compartment and the concentration compartment are both composed of a frame member having conduits that permit the passage of feed water, product water and concentrate water, and a plurality of such deionization and concentration compartments are stacked to fabricate an electrical deionization apparatus.
If one wants product water whose quality is comparable to that of ultra-pure water (with a specific resistance of at least 18 Mxcexa9xc2x7cm), pure water having a specific resistance of at least 5 Mxcexa9xc2x7cm is supplied to the concentration compartment and, optionally into an electrode compartment. Such pure water is conveniently obtained by branching a portion of the product water at the exit thereof. If desired, two or more units of the electrical deionization apparatus of the invention may be connected in series such that the concentration compartment and the electrode compartment of a unit is supplied with the water that has passed through the concentration compartment and an electrode compartment of the subsequent unit, with the concentration compartment and an electrode compartment of the last unit being supplied with deionized water having a specific resistance of at least 5 Mxcexa9xc2x7cm.